Core Hole States Research Articles

Carbon1score-hole lifetime inCO2

The carbon $1s$ photoelectron spectrum has been measured for ${\mathrm{CO}}_{2}$ at photon energies of 308, 320, and 330 eV with an instrumental resolution about half the natural linewidth. These spectra have been analyzed to obtain vibrational spacings, vibrational intensities, and the lifetime, \ensuremath{\tau}, of the carbon $1s$ core-hole state. Theoretical calculation of the lifetime width, \ensuremath{\Elzxh}/\ensuremath{\tau}, using a one-center model, which assumes that only the valence electrons localized on the atom with the core hole can participate in Auger deexcitation of the core hole, predicts a value of 66 meV, considerably smaller than that predicted for ${\mathrm{CH}}_{4}$ (96 meV). Experimental measurements indicate, however, that the ${\mathrm{CO}}_{2}$ carbon $1s$ width is, in fact, much larger than expected---$99\ifmmode\pm\else\textpm\fi{}2\mathrm{meV}$---and is approximately the same as that found experimentally for ${\mathrm{CH}}_{4}$ (95\ifmmode\pm\else\textpm\fi{}2 meV). This result indicates that valence electrons on the oxygen atoms may play a role in the Auger decay the carbon $1s$ core hole in ${\mathrm{CO}}_{2},$ and, hence, that a multicenter model may be necessary to describe the Auger process.

Core-valence coupling in the Ru4pphotoexcitation/Auger decay process: Auger-photoelectron coincidence spectroscopy study

The N{sub 23}VV Auger spectrum of Ru has been measured in coincidence with 4p{sub 1/2} and with 4p{sub 3/2} photoelectrons. Unlike other metals that exhibit bandlike Auger decays, we find that the two Auger spectra are not shifted by the difference in core level binding energies. A consistent description of these transitions and the core level line shape requires consideration of the relativistic multiplet splitting in the intermediate core hole state and two-valence-hole Auger final state. The results suggest that the large linewidth of the 4p levels is primarily due to multiplet splitting, and that an N{sub 2}(N{sub 3}N{sub 45})N{sub 45}N{sub 45} super-Coster-Kronig transition is only a minor decay channel. (c) 2000 The American Physical Society.

The electronic structure of rare-earth oxides in the creation of the core hole

We studied the electronic structures of rare-earth sesqui-oxides R 2O 3 (R: rare-earth element) with a core hole, both experimentally and theoretically. The experimental study was made by measuring the rare-earth 3d X-ray photoemission spectra (XPS) of these oxides. Clear double-peak structures were observed for La 2O 3, the shoulder or broad foot of the low-binding-energy side of 3d XPS were observed for Pr 2O 3, Nd 2O 3, and Sm 2O 3, and single-peak structures were observed for Eu 2O 3, Gd 2O 3, and Dy 2O 3. The calculation of electronic structures was performed for the ground state and the 3d –1 core-hole states by the spin-unrestricted DV-Xα molecular-orbital method for model clusters chosen as [RO 6] 9– for R 2O 3. The calculated results show the difference of the electronic structure in the ground state and the core-hole states. In the creation of the core hole, the number of unpaired 4f electrons increases by less than one due to the weak charge-transfer effect for La 2O 3 and Ce 2O 3, and this corresponds to the clear double-peak structure of the rare-earth 3d XPS. For Eu 2O 3 and Yb 2O 3, the number increases and decreases by less than 1 due to the restricted charge-transfer effect by filling 4f↑ and 4f orbitals with electrons. Due to the strong charge-transfer effect, the number of unpaired 4f electrons increase by more than 1 for Pr–Sm oxides, and this effect produces, the shoulder or broad foot in the low-binding-energy side of the rare-earth 3d XPS peak. The decrease by more than 1 for Gd–Tm oxides results in clear single peak structures. The present results indicate that rare-earth sesqui-oxides can be classified into the four groups by spin response.

Ion-stimulated desorption of O+ from the oxygenated TiC(111) surface: The role of nonadiabatic electronic transitions and the band effect

The mechanism of secondary O+ ion emission from the oxygenated TiC(111) surface has been investigated by noble-gas ion irradiation. The O+ ion is ejected from the surface by He+ irradiation due to an electronic transition without violent collisions. The O+ ion occurs preferentially from the weakly bound oxygen species, but very little O+ arises from the strongly chemisorbed oxygen on the threefold hollow site. The O+ emission is not caused by two localized valence holes resulting from the Auger decay of the primary ions, but is rather initiated by the formation of a long-lived O 2s core-hole state with an antibonding character. The O 2s hole is created by He+ via the nonadiabatic electronic transition during moderate impact-parameter collisions (1.0–3.0 a.u.), which is accompanied by very little kinetic energy transfer (<1 eV) to the target oxygen atom. The probability for O 2s hole creation increases with increasing velocity of the primary ions. The ionization of oxygen occurs due to the Auger decay of the O 2s hole on the way out from the surface after breakage of the chemisorptive bond; this is essential for the emission of very low-energy (<10 eV) O+ ions without resonant neutralization. This mechanism is consistent with that developed in low-energy ion scattering and successfully explains the O+ emission even from metal surfaces without the need for any assumption of the localization of valence holes.

Particle–hole interaction effect on a core-level XPS spectrum II

Abstract X-ray photoelectron spectroscopy (XPS) spectral behavior of an initial discrete core-hole state embedded or nearly embedded in a continuum is studied by a Green’s function model calculation. The particle–hole interaction U in the final two-hole one-particle state is included in the initial core-hole self-energy by the ladder and ring approximation. With an increase of U, the transition to an unoccupied (particle) state near the double-ionization threshold increases in the final state. With a further increase of U, the transition to an empty bound level below the Fermi level dominates. It is shown that even when the coupling between the initial discrete core-hole state and the final continuum state is strong there can be a sharp resonant state. When the coupling is weak, there is a possibility of the breakdown of the quasi-particle approximation for the initial core-hole state. U can substantially affect the initial core-hole spectral function.

Neutralization ofO+during scattering and ion-stimulated desorption from the TiC(100) surface

The mechanism of ion-stimulated ${\mathrm{O}}^{+}$ desorption from the oxygenated TiC(100) surface has been investigated, in comparison with the neutralization of ${\mathrm{O}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions during scattering. The ${\mathrm{O}}^{+}$ ion desorbs from the surface due to the potential energy of ${\mathrm{He}}^{+}.$ This process is initiated by two valence holes resulting from Auger decay of ${\mathrm{He}}^{+}$ but due to a long-lived O $2s$ core-hole state with the antibonding character, which is formed by the quasiresonant charge exchange with ${\mathrm{He}}^{+}$ $1s.$ In the ion-scattering experiment, the ${\mathrm{O}}^{+}$ ion is neutralized almost completely whereas ${\mathrm{Ne}}^{+}$ survives neutralization considerably even at energy as small as below 10 eV. This is because the lifetime of the core hole is much longer than that of the valence holes. The ab initio molecular-orbital calculation also reveals that the O $2p$ hole diffuses immediately into the band via the resonant tunneling process. The O $2s$ (or Ne $2p)$ core hole decays with a much longer lifetime via the two-electron Auger process. The core-excited oxygen species, which undergoes the intra-atomic Auger decay during and after breakage of the chemisorptive bond, results in the ${\mathrm{O}}^{+}$ ion.

Relaxation and decay of an atom with a core hole. III

The resonant core-electron excitation to a narrow empty band can result in a resonant state or a core-exciton-like bound state. The transition from the resonant state to the bound state depends on the relative size of the core-hole–excited-electron interaction strength to the band-width of the density of unoccupied states of a particular angular momentum character in the ground state. In a charge transfer system the hybridization of a localized empty acceptor-level with a donor band, results in a split-off bound level and/or a state in the band. The quasi-particle approximation could break down for a state in the band so that the concept of a resonant state becomes illusory. The core-hole state in which such a split-off bound level below the Fermi level is occupied by the resonant core-electron excitation or the core-hole screening electron, could become a fully relaxed core-hole state.

On the satellite in the Auger electron spectrum

The satellite line profile in the C–VV Auger electron spectroscopy spectrum depends upon how the initial core hole state and the final multiple hole state are created. Even if the numbers of the holes in the satellite states are the same, the Auger electron spectroscopy spectral profiles of the final multiple hole states are different. The localization of the three hole satellite state is discussed.

Resonant processes in the soft X-ray regime

Abstract The phenomenon of resonant excitation of X-ray and electron emission spectra is discussed. The basic phenomena are demonstrated for Ni metal. The importance of the localization of the intermediate state is discussed in connection with spectra for adsorbed rare gases. It is also discussed how dynamic information can be extracted from such results. For more strongly bonded adsorbate systems it is shown how resonant excitation can be used to prepare specific core-hole states for X-ray emission studies, still allowing a one-electron description of the final states.

Theoretical study of the4plevel photoemission spectra of Ba metal and related systems

The $4p$ hole spectral function of Ba metal is calculated by an ab initio atomic Green's-function method. The atom-metal energy shifts of the interacting hole (particle) states are taken into account. It is shown that for a consistent description of the spectral line positions, intensities, and linewidths, it is necessary to take into account explicitly such energy shifts. The $4p$ x-ray photoelectron spectroscopy spectra of Ba metal and compounds are analyzed in detail. The $2s\ensuremath{-}4p$ x-ray emission spectroscopy spectrum of La metal is also discussed. For the $4p$ level ionization of Ba metal and compounds, the one-electron approximation breaks down. This is due to the Coster-Kronig (CK) ${4p\ensuremath{-}4d}^{\ensuremath{-}2}4f$ process by which an empty $4f$ level is filled by a $4d$ electron rather than by a ligand valence electron [charge-transfer (CT) screening] or a conduction-band electron. A possibility of suppression of both CK and CT core-hole screening in a final state by decay of a resonantly excited core-hole state and a fully relaxed core-hole state where an $4f$ empty level is already occupied in an initial core-hole state is discussed.

Electronic structures of 3d transition metal (Ti–Cu) oxides probed by a core hole

Electronic structures of various 3d transition metal oxides are calculated using the spin-unrestricted discrete-variational (DV) Hartree–Fock–Slater (Xα) method for the ground state and 1s −1 core hole state. Model clusters MO n are used in place of calculating infinite size solids, where M is the transition metal elements from Ti ( Z=22) to Cu ( Z=29) and n the coordination number of oxygen; n=6 for O h symmetry and n=4 for T d or D 4h symmetry. The calculated results indicate that the number of unpaired 3d electrons reduces in the creation of the core hole for late transition metal compounds, i.e. do not represent the ground-state spin state for these compounds, but conserves for early transition metal compounds. These results provide theoretical basis on the validity of spin selective X-ray absorption spectroscopy for early transition metal compounds by using the high-resolution X-ray fluorescence yield method, and theoretical basis on the validity or nonvalidity of spin state analysis using core-level spectroscopy, such as X-ray emission, X-ray absorption, and photoelectron spectroscopies. It is also found that the spin of a class of compounds sometimes flips in the creation of the core hole.

Auger resonant Raman effect for dissociative core-excited states: General treatment and application to the HCl case

We apply our recently developed explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core hole states in diatomic molecules to the case of purely dissociative intermediate states and exemplify it by application to $\mathrm{Cl}{2p}_{3/2}$ excitation in HCl. A main goal is the conceptual understanding of the influence of detuning of the exciting radiation away from the excitation resonances. The calculated spectra for decay to different final states (binding as well dissociative ones, as well as to hypothetical final states selected to illustrate certain points of the behavior), are very rich; their main features agree with experimental observations and previous calculations using other approaches. The spectra are dominated by a sharp asymmetric peak which does not shift with photon energy. While its previous designation as ``atomic decay peak'' is warranted to first order, we show that it contains contributions from throughout the evolution in the intermediate state, i.e., also from times and internuclear distances for which the molecule has not yet separated. Its width is given by the core hole lifetime. The broad ``molecular decay'' contribution to the spectra can be very complicated, depending on the final state potential and the exciting radiation tuning conditions. Its width is determined mainly by the Franck-Condon width of the intermediate state wave packet. For strong detuning it approaches the direct photoemission spectral shape. When these two features overlap, deep spectral holes develop which we explain by interference of different paths (atomic and molecular decays) to the same final state. These and other findings are discussed in a conceptual way.

Separate state vs. transition state Kohn-Sham calculations of X-ray photoelectron binding energies and chemical shifts

The possibilities of using Kohn-Sham density functional theory to make accurate predictions of core photoelectron binding energies and chemical shifts are explored through a series of calculations on compounds of different sizes and types. The recent proposal of an ‘unrestricted generalized transition state’ (UGTS) method, for this purpose [D.P. Chong, Chem. Phys. Lett. 232 (1995) 486; D.P. Chong, J. Chem. Phys. 103 (1995) 1842], is put to test vs. full separate state optimizations of the ground and core hole states, that is a ‘ Δ Kohn-Sham’ ( Δ KS) method. It is found that while internal parametrizations in terms of grid and basis set expansions can be well controlled there is still a notable dependency of the absolute binding energies on the choice of functional for both the UGTS and Δ KS methods. As for pure ab initio Δ SCF the former method must still be viewed as an approximation of the latter. Keeping these dependencies in mind, Kohn-Sham calculations seem to provide a promising tool for predicting binding energies and chemical shifts of an accuracy that approaches that of experiments.

Dynamically Induced Spin Polarization of Resonant Auger Electrons

Resonant Anger electrons from the decay of the atomic Xe 4d(5/2)(-1)6p(J = 1) state excited with linearly polarized light are shown to have a strong spin polarization. The core hole states are not oriented in this case. We found polarization values ranging from -0.46 to 0.57. For the decay to the 5p(4)(D-1(2))6p(P-2(1/2)) state the ratio of the radiationless decay matrix elements and their relative phase, including its sign, are derived.

Detailed analysis of the3d−1→4pπ−2normal Auger spectra in HBr and DBr

High-resolution Br $\mathrm{MVV}$ Auger spectra of HBr and DBr have been recorded using monochromatized synchrotron radiation. We present a detailed analysis and reassignment of the spectra in the ${3d}^{\ensuremath{-}1}\ensuremath{\rightarrow}4p{\ensuremath{\pi}}^{\ensuremath{-}2}$ region. This part of the spectra exhibits the spin-orbit and ligand-field splitting of the ${3d}^{\ensuremath{-}1}$ core-hole states and a vibrational structure of the final states influenced by vibrational lifetime interference. The equilibrium distances and the vibrational energies of the final states ${}^{1}{\ensuremath{\Sigma}}^{+},$ ${}^{1}\ensuremath{\Delta}$, and ${}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ are derived from the intensity distribution of the vibrational substates. From the shape of the peaks identified as transitions to the ${}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ state a splitting of $\ensuremath{\cong}50$ meV between the components ${}^{3}{\ensuremath{\Sigma}}_{0}^{\ensuremath{-}}$ and ${}^{3}{\ensuremath{\Sigma}}_{1}^{\ensuremath{-}}$ is concluded. Agreement between the experimental spectra and fitted results for both HBr and DBr is very satisfactory.

Vibrationally resolved oxygen K→Π ∗ spectra of O 2 and CO

A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K→Π ∗ excitation in CO (534 eV) and O 2 (531 eV) molecules has been carried out. A Franck–Condon analysis of the transitions has been accomplished assuming a Morse potential energy curve for the core hole states. The vibrational constants of O 2 are in agreement with values expected from the equivalent core molecule OF. The values of the lifetime widths of the Π ∗ excited states are Γ O 2 =149±10 meV and Γ CO=158±8 meV. The measured lifetime width of O 2 is an upper limit to the true value as unresolved spin–orbit structure is believed to be present, which may cause an additional broadening of ∼5 meV. In spite of this, it is substantially smaller than the hitherto accepted value of 180 meV, necessitating a re-evaluation of most previously published data.

Carbon1sphotoelectron spectrum of methane: Vibrational excitation and core-hole lifetime

The carbon $1s$ photoelectron spectrum has been measured for ${\mathrm{CH}}_{4}$ at photon energies of 302, 320, and 330 eV and for ${\mathrm{CD}}_{4}$ at 330 eV with an instrumental resolution about half the natural linewidth. These spectra have been analyzed to obtain vibrational spacings, vibrational intensities, and the lifetime of the carbon $1s$ core-hole state. The vibrational intensities vary with photon energy, in agreement with earlier results. At 330 eV, the observed Franck-Condon factors for both ${\mathrm{CH}}_{4}$ and ${\mathrm{CD}}_{4}$ can be understood only if anharmonic effects (consistent with the predictions of theory) are included. On the other hand, the vibrational spacings in ${\mathrm{CH}}_{4}$ show no evidence for anharmonicity (in contrast with theoretical predictions). In ${\mathrm{CD}}_{4}$ the observed anharmonicity in the vibrational energy spacings is about half of the predicted value, but the experimental and theoretical values differ only by an amount comparable to the experimental uncertainty. The measured values of the lifetime show a dependence on photon energy; this is attributed to failure of the theory of post-collision interaction to predict correctly the observed electron spectrum near threshold. At 330 eV, the measured Lorentzian lifetime, 93--95 meV, agrees with predictions of simple theory, but not with the prediction of more complete theory. It is also observed that there are systematic discrepancies between the observed line shapes and those predicted by the theory of postcollision interaction.

In situ XAFS studies of the adsorption of benzene in zeolite beta: differences between fluorescence and electron yield detection

By using a newly designed cell to perform in-situ XAFS at the A1 K edge the adsorption of benzene in Beta zeolite is studied. The data were collected both in fluorescence and electron yield mode. The adsorption of benzene changed the near edge structure of the AI K edge in Beta-zeolite. These changes are much larger when the data are collected in the electron yield mode. A broad satellite peak has been observed in the electron yield data. This peak can be assigned to a core-hole 2-electron final state. The 2electron final state occurs since in addition to the excited electron another electron is transferred from the benzene to the 3p orbital of the AI absorber atom (charge transfer, CT). In fluorescence mode this peak is much less pronounced, because the twoelectron state decays primarily via Auger decay.

Local spin moment of LaCoO 3 probed by a core hole

The electronic structures of the ground state and the 1s −1 core-hole state of high-spin cobalt oxide clusters are calculated. The calculated results indicate that the high-spin Co 3+ oxide becomes low spin in the core-hole state. To check the validity of the present calculation, we have measured the high-resolution X-ray fluorescence spectra of the Co K–L 3 (Kα 1) line shape for various cobalt compounds using a double-crystal X-ray fluorescence spectrometer: Co(acac) 2, Co(acac) 3, CoF 3, CoCl 2, CoBr 2, K 3[Co(CN) 6], CoO, Co 3O 4, LaCoO 3, and Co (metal), where acac denotes CH 3COCHCOCH 3 −. We conclude from the comparison of the calculations with the experimental results that the cobalt ion in LaCoO 3 at room temperature is high-spin state in spite that the reported spin state of this perovskite is different among the probing methods. The implications of the spin-flip shake-down process for the interpretation of the core-level X-ray photoelectron spectra, X-ray absorption spectra, and X-ray fluorescence spectra are discussed.

3s photoemission spectra of 3d-transition-metal atoms on graphite

The 3s photoemission spectra of 3d-transition-metal atoms adsorbed on graphite are calculated with a cluster model. In addition to exchange splitting, the spectra show charge transfer satellites from final state core hole screening by the graphite π-band. For the late 3d elements, exchange and charge transfer satellites are strongly mixed, which leads essentially to a three peak structure of the spectra.